Plagioclase in Yamato-7308 Meteorite from Antarctica

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概要

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• Twinning patterns, optical angles, Kohler angles, and other optical properties and chemical compositions of the calcic plagioclases in Yamato-7308 meteorite from Antarctica were examined. The Yamato-7308 meteorite is a howardite consisting of polymict breccia of eucrite and diogenite fragments. Plagioclases occur as individual grains in matrix, as a main constituent of eucrite fragment, and as maskelynite in eucrite fragment. These plagioclases have petrographical characteristics exhibiting their different petrogenetical processes. Plagioclase grains in eucrite fragment are twinned after the albite-Carlsbad, albite and pericline laws, and show rarely the Carlsbad law. Frequency percentage of the albite-Carlsbad and Carlsbad laws reaches 45%. Average composition of the plagioclase is Or_<0.2>Ab_<10.3>An_<89.4>. Individual plagioclase grains in matrix are twinned after the pericline and albite laws, and the albite-Carlsbad law is rare. Frequency percentage of the pericline law reaches 76%, and that of the albite-Carlsbad law is only 4%. Average composition of the individual plagioclase grains is Or_<0.1>Ab_<7.9>An_<92.0>. The twinning pattern of plagioclases in eucrite fragment clearly differs from that of individual plagioclase grains in matrix. Concerning the twinning pattern of plagioclase, Yamato-7308 meteorite contains two contrastive kinds of plagioclases : igneous type (in eucrite fragment) and metamorphic type (in matrix). These plagioclases are considered to have been formed under the same genetical environment. Regional metamorphism occurred in an asteroid after its eucrite crust and diogenite mantle were formed. Individual plagioclase grains were formed by the regional metamorphism and eucrite fragments retain their original igneous characteristics owing to their situation far from the metamorphic terrain or their resistance to the regional metamorphism. Howardite was formed by collision between the partly metamorphosed asteroid and some meteoritic body. Some individual plagioclase grains show clear shadow extinction and kink bands, and plagioclase in some eucrite fragments is maskelynitized either completely or locally. These facts indicate that some individual grains and some rock fragments have been subjected to intense impact metamorphism by the collision.

• 国立極地研究所の論文

著者

• Suwa Kanenori

  Department Of Earth And Planetary Sciences School Of Science Nagoya University

• Yagi Kenzo

  Hokusei Gakuen College

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